Fiber-reinforced syntactic foam composites prepared from polyglycidyl aromatic amine and polycarboxylic acid anhydride

ABSTRACT

Fiber-reinforced syntactic foam composites having a low specific gravity and a coefficient of thermal expansion of about 9.0×10 -6  in/in/°F. (16.2×10 -6  cm/cm/°C.) or less are prepared from a mixture of: a heat curable thermosetting resin comprising an uncured polyglycidyl aromatic amine, a polycarboxylic acid anhydride curing agent, and a chosen curing accelerator; hollow microspheres having a diameter of about 5 to 200 micrometers; and fibers having a length less than or equal to 250 micrometers. These composites are useful for forming lightweight structures for space applications.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, in general, to syntactic foam compositesand, more paricularly, to fiber-reinforced thermosetting resin basedsyntactic foam composites exhibiting a low specific gravity and a lowcoefficient of thermal expansion.

2. Description of the Prior Art

A continuing objective in the development of satellites is to optimizesatellite payload weight. One means of achieving this objective is toreduce the intrinsic weight of various operational elements within thespacecraft. It has been recognized by the art that the desired weightreduction could be realized by replacing conventional materials, such asaluminum, with lower density synthetic composites possessing requisitemechanical, thermal and chemical stability. Included in these lowdensity synthetic composites is a group of materials referred to in theart as syntactic foams.

Syntactic foams are produced by dispersing microscopic rigid, hollow orsolid particles in a liquid or semi-liquid thermosetting resin and thenhardening the system by curing. The particles are generally spheres ormicroballoons of carbon, polystrene, phenolic resin, urea-formaldehyderesin, glass, or silica, ranging from 20 to 200 micrometers in diameter.Commercial microspheres have specific gravities ranging from 0.033 to0.33 for hollow spheres and up to 2.3 for solid glass spheres. Theliquid resins used are the usual resins used in molding reinforcedarticles, e.g., epoxy resin, polyesters, and urea-formaldehyde resins.

In order to form such foams, the resin containing a curing agenttherefor, and microspheres may be mixed to form a paste which is thencast into the desired shape and cured to form the foam. The lattermethod, as well as other known methods for forming syntactic foams, isdescribed by Puterman et al in the publication entitled "Syntactic FoamsI. Preparation, Structure, and Properties," in the Journal of CellularPlastics, July/August 1980, pages 223-229. When fabricated inlarge-block form, such foams possess a compressive strength which hasmade them suitable for use in submerged structures. In addition, themore pliable versions of the foam are utilized as filler materialswhich, after hardening, function as a machinable, local-densificationsubstance in applications such as automobile repair and the filling ofstructural honeycombs. Despite these characteristics of adequatecompressive strength, good machineability, and light weight, such foamslack the degree of dimensional and thermal stability required to renderthem applicable for the spacecraft environment. More specifically,syntactic foam systems tend to exhibit varying filler orientation anddistributions within the geometrical areas in a molded intricatestructure, which limits the structural intricacy that can be achieved,as well as reducing dimensional stability. If syntactic foam systems aretoo highly filled, sacrifices are made in moldability, coefficient ofthermal expansion, strength, density, dimensional stability andstiffness. Moreover, such foams tend to exhibit poor adhesion tometallic plating which is required to form the desired product, such asan antenna component.

In order for the syntactic foam to be useful as a substitute foraluminum in antenna and antenna microwave components in a spacecraft,the foam must have the following characteristics.

(1) The material must have a specific gravity of 1.00 or less, ascompared to a specific gravity of 2.7 for aluminum.

(2) The material must have a linear coefficient of thermal expansion (αor CTE) comparable to that of aluminum, preferably close to 13×10⁻⁶in/in/°F. (23×10⁻⁶ cm/cm/°C.) or less. Thermal distortion of antennacomponents subjected to thermal cycling in the extremes of the spaceenvironment is a major contributing factor to gain loss, pointingerrors, and phase shifts.

(3) The material must meet the National Aeronautics and SpaceAdministration (NASA) outgassing requirements to insure that thematerial does not release gaseous component substances which undesirablyaccumulate on other spacecraft parts in the outer-space vacuum.

(4) The material must have long-term stability, as required for partsexposed to the space temperature environment (e.g., -100° F. to 250° F.or -73° C. to 121° C.) for extended periods of time, such as 10 years.

(5) The material must be capable of being cast into complexconfigurations in order to form component parts for antenna structures,such as waveguides or antenna feed distribution networks.

The art, until the present invention, has been unable to satisfy theserequirements and particularly the requirement for a low coefficient ofthermal expansion (α). Thus, known epoxy resin based syntactic foamsfilled with 10 to 30% by volume hollow microspheres generally have a αin the range of 17 to 36×10⁻⁶ in/in/°F. (30 to 65×10⁻⁶ cm/cm/°C.).

A need, unsatisfied by existing technology, has thus developed for asyntactic foam material which is both lightweight and of sufficientmechanical, thermal and chemical stability to enable it to besubstituted for aluminum in physically demanding satellite environments.

SUMMARY OF THE INVENTION

The unresolved needs of the art are satisfied by the present inventionwhich provides thermally stable fiber-reinforced syntactic foamcomposites having a specific gravity of less than 1.0 and a linearcoefficient of thermal expansion of about 9.0×10⁻⁶ in/in/°F. (16.2×10⁻⁶cm/cm/°C.) or less, which are prepared from an admixture of: a heatcurable thermosetting resin comprising an uncured polyglycidyl aromaticamine, a polycarboxylic acid anhydride curing agent, and a chosen curingaccelerator; hollow microspheres having a diameter between about 5 andabout 200 micrometers; and fibers having a length of about 50 to about250 micrometers.

The syntactic foam composites of the present invention can be cast ascomplex structures which contain lightweight hollow microspheres havingfibers, such as graphite fibers, in the voids between the microspheres,with the microspheres and fibers being bonded together by the heat curedresin matrix. The composites of the present invention readily meet thespecific gravity, coefficient of thermal expansion and NASA outgassingrequirements, which easily qualify the composites as aluminumsubstitutes for spacecraft use.

DETAILED DESCRIPTION OF THE INVENTION

In order to form the fiber-resin-microsphere composite of the presentinvention having the desired density and coefficient of thermalexpansion, each of the three components must be selected so that theresulting combination thereof provides a mixture amenable to being castinto the desired configuration, as well as providing a final producthaving the required structural and physical properties. Acceptablemixtures must have a viscosity that produces an accurate, voidfreecasting with uniform material properties. In addition, the proportion offiber in the composite must provide the required thermal expansion,strength, and stiffness properties. Further, the microsphere componentmust be chosen to provide the required low density in the composite.Finally, each of the components must be capable of being combined withthe other components and the effect of each on the other in the mixturethereof, as well as in the final composite must be taken into account.In particular the properties of the composite are influenced by theproperties, relative volume ratios, and interactions of the individualcomponents. More specifically, density, strength, stiffness(brittleness), coefficient of thermal expansion and processibility arestrong functions of filler and fiber type, volume ratios andmicropacking. The following discussion provides a more detailedconsideration of these various factors. It should be noted that in thefollowing discussion, the term "syntactic foam" is used herein to denotea filled polymer made by dispersing rigid, microscopic particles in afluid polymer or resin and then curing the resin, as is known in theart. The term "fiber-reinforced syntactic foam composite" is used hereinto denote the cured product formed from the mixture of resin,microballoons, and reinforcing fibers in accordance with the presentinvention.

1. Heat Curable Resin

The heat curable, thermosetting resins used to prepare the syntacticfoam composites of the present invention can be any heat curablethermosetting resin having appropriate viscosity for casting (e.g., lessthan 1000 centipoise), pot life (e.g., greater than 2 hours),coefficient of thermal expansion, and thermal stability in thetemperature range of -100° F. to 250° F. (-73° C. to 121° C.) requiredin the space environment. The resin material contains a curing agentwhich reacts with the resin to produce a hardened material. Curingagents and other additives will, of course affect the viscosity andother properties of the final mixture from which the composite isformed. Examples of suitable resins include low viscosity, polymerizableliquid polyester resins which comprise the product of the reaction of atleast one polymerizable ethylenically unsaturated polycarboxylic acid,such as maleic acid or its anhydride, and a polyhydric alcohol, such as,for example, propylene glycol and optionally, one or more saturatedpolycarboxylic acids, such as, for example, phthalic acid or itsanhydride. Other suitable resins include condensates of formaldehydesuch as ureaformaldehyde, melamine-formaldehyde and phenolformaldehyderesins. Preferred resins for use in the practice of the presentinvention are epoxy resins having 1,2 epoxy groups or mixtures of suchresins, and include cycloaliphatic epoxy resins such as the glycidylethers of polyphenols, liquid Bisphenol-A diglycidyl ether epoxy resins(such as those sold under the trademarks Epon 815, Epon 825, Epon 828 byShell Chemical Company), phenolformaldehyde novolac polyglycidyl etherepoxy resins (such as those sold under the trademarks DEN 431, DEN 438and DEN 439 by Dow Chemical Company), and epoxy cresol novolacs (such asthose sold under the trademarks ECN 1235, ECN 1273, ECN 1280 and ECN1299 by Ciba Products Company).

The particular epoxy resins preferred in the practice of the presentinvention are polyglycidyl aromatic amines, i.e. N-glycidyl aminocompounds prepared by reacting a halohydrin such as epichlorohydrin withan amine. Examples of the most preferred polyglycidyl aromatic aminesinclude diglycidylaniline, diglycidyl orthotoluidine, tetraglycidylether of methylene dianiline and tetraglycidyl metaxylene diamine, ormixtures thereof.

The epoxy resins which are preferably in liquid form at room temperatureare admixed with polyfunctional curing agents to provide heat curableepoxy resins which are cross-linkable at a moderate temperature, e.g.,about 100° C., to form thermoset articles. Suitable polyfunctionalcuring agents for epoxy resins include aliphatic polyamines of whichdiethylene triamine and triethylene tetramine are exemplary; aromaticamines of which methylene dianiline, meta phenylene diamine, 4,4'diaminodiphenyl sulfone are exemplary; and polycarboxylic acidanhydrides of which pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride, hexahydrophthalic anhydride, nadic methylanhydride (maleic anhydride adduct of methyl cyclopentadiene), methyltetrahydrophthalic anhydride and methyl hexahydrophthalic anhydride areexemplary. Polycarboxylic acid anhydride compounds are preferred curingagents for the above-noted preferred epoxy resins, with the threecompounds last noted being most preferred.

In preparing heat curable, thermosetting, epoxy resins compositions, theepoxy resin is mixed with the curing agent in proportions from about 0.6to about 1.0 of the stoichiometric proportions, which providessufficient anhydride groups and carboxylic acid groups to react withfrom about 60 to 90 percent of the epoxide groups. The term "curing" asused herein denotes the conversion of the thermosetting resin into aninsoluble and infusible cross-linked product and, in particular, as arule, with simultaneous molding to give shaped articles.

In addition curing accelerators may be added to the epoxy resins, as isknown in the art, to provide a low curing temperature. Preferredaccelerators for the above-noted preferred polyglycidyl aromatic amineresins are substituted imidazoles, such as 2-ethyl-4-methyl imidazole,and organometallic compounds, such as stannous octoate, cobalt octoate,and dibutyl tin dilaurate which are incorporated at a concentration ofzero to about 3 parts by weight per 100 parts resin.

Moreover, other materials may be added to the epoxy material in order toimprove certain properties thereof, as is known in the art. For example,the tendency of the resin to separate from the mixture can be minimizedby the addition of fine particulate fillers, such as Cab-O-Sil (a fumedsilica manufactured by Cabot Corporation), acicular fibers, such astalc, or short chopped or milled fibers. In addition, resin penetrationof the filler may be enhanced by the addition of a titanate wetting,agent, such as KR38S, an isopropyl tri(dioctylpyrophosphate)titanate,available from Kenrich Petrochemical Co.

A particularly useful resin composition for forming the composites ofthe present invention comprises a polyglycidyl aromatic amine, apolycarboxylic acid anhydride curing agent, and a curing accelerator, asdescribed in copending patent application Ser. No. 608615, filed May 9,1984, assigned to the present assignee. Examples 3 and 4 herein aredirected to the use of this preferred resin formulation in the practiceof the present invention.

2. Hollow Microspheres

The syntactic foam composites prepared in accordance with the presentinvention contain a relatively uniform distribution of hollowmicrospheres. These hollow microspheres are usually hollow thermoplasticspheres composed of acrylic-type resins such as polymethylmethacrylate,acrylic modified styrene, polyvinylidene chloride or copolymers ofstyrene and methyl methacrylate; phenolic resins; or hollow glass,silica or carbon spheres that are very light in weight and act as alightweight filler in the syntactic foam. These microspheres preferablyhave a diameter in the range of about 5 to about 200 micrometers.Methods for the production of these hollow microspheres are well knownin the art and are discussed, for example, by Harry S. Katz and John V.Milewski in the book entitled, "Handbook of Fillers and Reinforcementsfor Plastics," Chapter 19: Hollow Spherical Fillers, Van NostrandReinhold, 1978, the teachings of which are incorporated herein byreference. Such microspheres are readily available commercially. Thesehollow microspheres can be compressed somewhat when subjected toexternal pressure. However, they are relatively fragile and willcollapse or fracture at high pressures. Therefore, there is a pressurerange under which the microspheres can effectively operate. It has beendetermined that when hollow glass microspheres are employed in thepractice of the present invention, syntactic foam composites can bemolded at pressures up to the limit of the hollow microspheres withoutfracture, with molding pressures in the range of about 700 to about 900psi (0.102 to 0.131 pascals) being preferred.

By controlling the amount of hollow microspheres added to the syntacticfoam, it is possible to control the specific gravity of the foam. Asimple mixture of an epoxy material and hollow microspheres tends toseparate on standing, with the microballoons rising to the surface ofthe epoxy. However, it has been found that with an increased volume ofmicroballoons added to the epoxy, there is a decreased tendency toseparate into discrete phases. Moreover, it has been found that at asufficiently high loading of microballoons, namely about 65% by volumefor microballoons, the tendency to separate into discrete phases isminimized. To achieve specific gravities of less than 1.0, the hollowmicrospheres are included in the syntactic foam in up to 65% by volumeand generally in a range of about 35 to about 65% by volume andpreferably about 50 to about 65% by volume. The volume percentage ofhollow microspheres is adjusted based on the composition of the hollowmicrosphere selected, the brand of microspheres and the size of themicrospheres. Therefore, it may be necessary to select the propermixture of heat curable resin material and hollow microspheres forpreparation of the syntactic foam on a trial and error basis. Forexample, the C15/250 series of glass microspheres available from the 3MCompany has a specific gravity of 0.15 and a mean diameter of 50micrometers. "Carbosphere" carbon microspheres available from the VersarCorporation have a specific gravity of 0.32 and a mean diameter of 50micrometers. Desirably, a mixture of two or more types of hollowmicrospheres may be employed in the practice of the present invention.The glass microspheres provide the syntactic foam with improvedstructural strength, while those of carbon advantageously contribute toboth a lowered coefficient of thermal expansion and greater amenabilityto subsequent metal-plating operations. When using a combination ofglass and carbon microspheres in preparing the composites of the presentinvention, the ratio of glass microspheres to carbon microspheres isabout 1:4 to 1:1.

Furthermore, it has been found by using packing theory that an increasedvolume percent solids in the resin mixture can be achieved. Packingtheory is based on the concept that, since the largest particle sizefiller in a particular reinforcement system packs to produce the grossvolume of the system, the addition of succeedingly smaller particles canbe done in such a way as to simply occupy the voids between the largerfiller without expanding the total volume. This theory is discussed byHarry S. Katz and John V. Milewski, in the book entitled "Handbook ofFillers and Reinforcements for Plastics," Chapter 4. Packing Concepts inUtilization of Filler and Reinforcement Combinations, Van NostrandReinhold, 1978. The fillers used in the present invention are chosen onthe basis of particle size, shape, and contribution to overall compositeproperties. This theory applies to the use of solid particulates as wellas hollow spheres. Because of the high viscosity of such a highly loadedresin, the mixture could not flow into the mold without damaging themicrospheres. To overcome this problem, the mold is pre-packed with thedry filler (i.e. a mixture of microspheres and fibers). By applyingpacking theory as described above, the filler can be packed at highdensity and so that segregation of ingredients does not occur.

Finally, the microspheres may be advantageously treated with a couplingand wetting agent to enable the resin to wet the sphere surfaces andpromote good filler-resin adhesion, as discussed in greater detail belowwith regard to similar treatment of the fibers used in the presentinvention.

3. Fibers

The fibers used in the practice of the present invention must becompatible with the selected resin in order to provide good couplingbetween the fiber and resin. Fibers such as graphite, glass, Kevlar (anaromatic polyamide material obtained from E. I. Dupont and Company)nylon, or carbon are added to the syntactic foam composite of thepresent invention to improve the strength and dimensional stability ofthe composite. However, the contribution of the fiber to the coefficientof thermal expansion of the composite product and to the viscosity ofthe mixture of components must also be considered. Graphite fibers havebeen found to be particularly useful since they provide the desiredstrength in the composite, while also reducing the coefficient ofexpansion of the composite. An additional factor to consider is fiberlength. While shorter fibers (e.g. having a length-to-diameter ratio ofless than 100:1) provide less reinforcement per fiber than do longerfibers, shorter fibers have less impact on the viscosity of the mixture.Thus, a greater volume fraction of shorter fibers can be incorporatedinto a mixture at a given level of viscosity, which provides a higherlevel of reinforcement at that viscosity level by shorter fibers. Inaddition, the use of shorter fibers improves the uniformity of the mix.Thus, fibers useful in the composite of the present invention have alength less than or equal to 250 micrometers and generally in the rangeof about 50 to about 250 micrometers. Fibers having a length about 150to about 250 micrometers were found to provide the best compromisebetween viscosity and reinforcement as discussed previously. Whengraphite fiber, the preferred fiber material, is used, the diameter ofthe graphite fibers is in the range of about 5 to about 10 micrometers.

Moreover, the interaction of the fibers with the microspheres discussedpreviously must be considered. It has been determined by micropackingtheory, as described in Chapter 4 of the book by Katz and Milewski,previously referenced, that the optimum ratio of fibers-to-spheresvaries with the length/diameter ratio (L/D) of the fibers and with theratio of the sphere-diameter to the fiber-diameter (R). For each valueof L/D, there is one R value where the packing efficiency is zero; andas R increases or decreases on either side of this minimum, packingefficiency increases. It has been found most desirable in the practiceof the present invention to use graphite fibers of the micrometerlengths discussed above, which have a length to diameter ratio (L/D) ofabout 5:1 to about 30:1 and preferably about 15:1 to about 30:1, and asphere-diameter to fiber-diameter ratio (R) of at least about 6:1 andpreferably about 15:1.

Graphite fibers used in the practice of the present invention areselected to have high strength and low density. Celanese GY-70 graphitefiber and Courtaulds HM-S graphite fiber are especially suitable.Celanese GY-70 fiber is 8 micrometers in diameter, has a tensilestrength of 76,000 pounds per square inch (3.6389×10⁶ Pa), a specificgravity of 1.83 gm/cm³ and an α of -0.3×10⁻⁶ in/in/°F. Courtaulds HM-Sgraphite fibers have a diameter of 8 micrometers, a tensile strength of50,000 psi (2.394×10⁶ Pa), a specific gravity of 1.91 gm/cm³ and alongitudinal α of -1×10⁻⁶ in/in/°F. The graphite fibers are commerciallyavailable as continuous-fiber tows. For example, Celanese GY-70 fiberconsists of 384 fibers/tow. The fiber tows are reducible to requiredlengths on the order of between about 50 micrometers and 250 micrometersby ball milling or from commercial processing concerns such as theCourtaulds Company of the United Kingdom.

The amount of fiber incorporated in the resin-microsphere admixturegenerally ranges from about 3 to about 10 volume percent and preferablyfrom about 3 to about 5 volume percent in order to achieve compositeshaving α values of 25×10⁻⁶ in/in/°F. (45×10⁻⁶ cm/cm/°C.) or less.

As the amount of hollow microspheres and fibers incorporated in the heatcurable resin increases, there is a corresponding increase in theviscosity of the resin. High viscosity prevents uniform dispersion ofthe microspheres and fibers and interferes with the processing of theresin-microsphere-fiber mixture during molding operations. However, inorder to reduce the viscosity of the mixture, the surfaces of themicrospheres and fibers may be provided with a thin layer of couplingand wetting agents. The microsphere and fiber surfaces are treated witha solution containing a silane coupling agent such as Silane A-186(beta(-3,4-epoxy cyclohexyl)-ethyltrimethoxy silane), Silane A-1120(n-beta-(aminoethyl)-gamma-aminopropyl tri-methoxy-silane) or a titanatecoupling agent such as di(dioctylpyrophosphato)ethylene titanate (KR238Mavailable from Kenrich Petrochemical Company of Bayonne, N.J.); ortetra(2,2 diallyloxymethyl-1-butoxy)titanium di(ditridecyl phosphite)(KR55 available from Kenrich); or titanium di(cumylphenylate)oxyacetate(KR134S available from Kenrich); or isopropyl tridodecylbenzenesulfonyl(KR9S available from Kenrich). The coupling agents enable the resin towet the sphere and fiber surfaces, and promote a stronger bond betweenthe resin, microspheres, and fibers without increasing the viscosityappreciably.

The coupling agents may be applied by simply dissolving the agents inthe resin-microsphere-fiber blend. Optionally, these agents may beapplied by first dissolving the agents at a concentration of 0.1-0.5% ofthe filler weight in water or an organic solvent such as isopropanol orFreon TE (a fluorocarbon compound available from E. I. Dupont andCompany); and then immersing the microspheres and fibers which have beenpremixed in predetermined proportions in the solution for a period of 5to 30 minutes, followed by filtering and drying the mixture. Themicrosphere-fiber mixture may then be blended with the heat curableresin preparatory to fabricating the syntactic foam composite.

4. Optional Microbeads

Solid microbeads may optionally be incorporated in the composite of thepresent invention in order to increase packing efficiency.Advantageously, such microbeads were also found to decrease theviscosity of the formulation, improve its pourability, and increasecomposite uniformity. In a preferred practice of the present invention,about 2 to about 8 percent by volume of solid inert material, such asglass or silica microbeads having a diameter of about 2 to about 8micrometers and a specific gravity of 2.2 to 2.4 are incorporated in theresin-microsphere-fiber admixture. Volume percentages in excess of 8%increase the viscosity of the uncured, filled heat curable resinformulation to a level at which it is unworkable for molding purposes.In addition, it was found that large filler volume fractions (volume ofmicroballoons, fiber and microbeads greater than 60 percent) had areduced coefficient of thermal expansion, but the viscosity of the mixwas unworkable. Small volume fractions of filler (i.e. volume ofmicroballoons, fiber, and microbeads less then 40 percent) were found toimprove processability, but increased the coefficient of thermalexpansion to an unacceptable level. However, by choosing a fillercombination that maximized filler volume yet minimized filler surfacearea, both viscosity and the coefficient of thermal expansion werereduced. Such a combination was used in the reinforced syntactic foamRSF-34F shown in Table III, which was processable, uniform, had goodphysical properties, and was successfully cast in a metal mold.

In preparing syntactic foams in accordance with the present invention,the hollow microspheres and graphite fibers, and optionally the solidmicrobeads, are admixed with the heat curable resin in any conventionalfashion using a suitable mixing device such as a Waring blender. Thehomogeneous admixture is then degassed as by applying a vacuum. Then themixture is loaded into a mold of suitable configuration from a reservoiror by using an air gun or other conventional loading device. The shapeof the mold will, of course, determine the shape of the cured productand may be chosen as required to form a desired structure, such as anantenna waveguide. Molding is then accomplished in an autoclave at thetemperature at which the resin is curable, e.g. to 250° F. to 350° F.(121° C. to 177° C.), for epoxy resins generally and about 150° F. to250° F. (66° to 121° C.) for the preferred epoxy composition describedherein, at 50 to 100 psi (2586 to 5171 mm Hg or 7.25 to 14.5×10⁻³ Pa)for about 2 to about 4 hours.

Molding of the filled heat curable resin formulations to form syntacticfoam composites of the present invention may also be effected by otherconventional molding methods including transfer molding and compressionmolding procedures wherein the heat curable formulation is cured at theabove-noted curing temperatures, using pressures on the order of 800 to1000 psi (41,372 to 51,715 mm Hg or 0.116 to 0.145 pascals) for 1 to 2hours.

It has been found particularly advantageous to form the filled heatcurable resin mixtures into the syntactic foam composites of the presentinvention by the vacuum liquid transfer molding process disclosed incopending patent application Ser. No. (PD-83306), assigned to thepresent assignee. In this procedure, the mold is first loaded with themicrosphere/fiber filler which has been mechanically or manuallypremixed in predetermined proportions and pretreated with a sizing agentas previously described. Next, the mold may optionally be vibrated topromote a uniform distribution of the filler in the mold (e.g. about 5minutes on a vibration table). Then the mold cavity is filled with theheat curable resin. The mold is a sealable pressure vessel constructedto support the vacuum/pressure sequence described below. To prepare forthe molding process, the mold cavity is preheated to bring the cavity upto the temperature at which the heat curable resin is curable. A vacuumis then drawn on the mold to degas the mold cavity contents and toimpregnate the filler with the resin. The vacuum is released toatmospheric pressure to burst any gas bubbles remaining in the moldcontents. Then, a superatmospheric pressure, such as 100 to 1000 psi(0.01456 to 0.145 pascals), is applied to the mold to cause the resin toencapsulate the filler. The elevated temperature and superatmosphericpressure are maintained for a time sufficient to partially cure theresin and form a unitary structure which can be ejected from the mold.The ejected structure is then subjected to a further heating cycle tocompletely cure the resin.

By the practice of the present invention, reinforced syntactic foamcomposites are obtained which have a coefficient of thermal expansion ofabout 9.0×10⁻⁶ in/in/°F. (16.2×10⁻⁶ cm/cm/°C.) or less and a density ofless than 1.0 cm³, as well as long-term thermal stability, amenabilityto being molded in various configurations, and ability to meet the NASAoutgassing requirements. In addition, the mechanical properties of thesecomposites are repeatable. This combination of properties makes thecomposites of the present invention particularly well suited for use asa substitute for aluminum in antenna and antenna microwave componentsused in space applications. In particular, heat curable epoxy resinscomprised of mixtures of tetrafunctional aromatic epoxy resins andliquid anhydride when heated to 150° F. are sufficiently low inviscosity to accept loadings of microspheres up to 65 percent of thevolume of the system, fiber loadings of up to 10 percent, and beadloadings up to 65 percent. These microsphere/fiber/bead filled epoxyresins are readily curable and when cured produce syntactic foamcomposites having specific gravities of between 0.8 and 0.9 andcoefficients of thermal expansion approximating that of aluminum orsteel. Depending on the filler fiber volume used in the composite of thepresent invention, composites may be tailored to have coefficients ofthermal expansion ranging from that of the unfilled resin to that ofsteel.

Because of their relatively low coefficient of thermal expansion, epoxyresin based syntactic foam composites prepared in accordance with thepresent invention have been determined to be especially amenable toconventional metal plating processes, such as electroless plating, whenthe surfaces thereof are prepared for plating by plasma treatment. Therelatively high adhesion of metal deposits to the surface of the presentcomposite is believed to be a function of both the topography of theplasma-treated surface plus the mechanical integrity of the remainingsurface. The plasma removes the resin "skin" from the composite, leavingthe graphite fiber/microballoon filler exposed, to provide a surfacewhich is readily platable. Such metal plating of the composite of thepresent invention may be required in forming antenna components in whichan electrically conductive surface or path is required, as is known inthe art.

To effect plasma treatment in preparation for plating, the surface ofthe filler reinforced epoxy resin based composite is subjected to aplasma process with a reaction gas containing a mixture of air,nitrogen, or argon with oxygen, water vapor, nitrous oxide, or otherresin oxidizing source, to remove the polymer "skin" and expose thefiller, as discussed above. Normal plasma etching conditions known tothe art are used. For example, for a plasma excitation energy of 200watts/ft² of composite, an O₂ /inert gas source of approximately 1000ml/minute, a vacuum pressure of 200 micrometers Hg, and one hourduration are used.

When a silver deposit is required, as in an antenna waveguide structure,it is advantageous to first form a layer of an electroless or vapordeposited metal such as copper to provide a conductive surface which canthen be built up with additional electrolytic plating such as copper orsilver plate to produce a smooth surface finish. Electrolytic silverplating may readily be formed on the electrolytic copper surface toprovide a silver plated surface with good adhesion to the underlyingcomposite material.

Electroless plating of the plasma-treated composite surface can beaccomplished by standard procedures such as by dipping theplasma-treated composite in the plating solution for a time sufficientto achieve a continuous buildup of metal on the etched surface. Metalsthat can be plated on the molded epoxy resin based composites preparedin accordance with the present invention include, for example, copper,silver, nickel, cobalt, nickel/iron, nickel/cobalt, other nickel alloys,and gold. For electroless copper plating, an aqueous bath of Shipley Co.™328 copper plating solution may be used, which contains copper sulfate,sodium potassium tartrate, and sodium hydroxide. Other electrolesscopper plating formulations can also be employed. The plating bath isagitated or stirred prior to immersion of the plasma-treated composite.Preferred plating temperatures are in the range of about 15° C. to about95° C. (about 59° F. to 203° F.). Metal adhesion of this electrolesscopper plating has been determined to be excellent even after exposureof the plated composite to cycles of widely different temperatures, asdescribed in Examples 3 and 4 herein.

Next, a copper plating is built up to any desired thickness on theelectroless copper by known electrolytic plating methods, usingcommercially available electrodeposit copper plating solution. Finallyan electrolytic silver plate is formed to the desired thickness on theelectrolytic copper plate by known methods, using commercially availablesilver plating solution formulations. Silver plating of a composite ofthe present invention is described in Example 5.

The following examples illustrate but do not limit the presentinvention.

EXAMPLE 1

This example illustrates a process for forming one type offiber-reinforced syntactic foam composite which is related to thepresent invention and which is described in copending patent applicationSer. No. 607,847, filed May 7, 1984 assigned to the present assignee.

The components of the syntactic foam formulation designated "S-61" areshown in Table I. The following details regarding the components of S-61apply to Table I.

a. MY720 is a tetraglycidyl methylene dianiline manufactured by CibaGeigy.

b. HY906 is a nadic methyl anhydride hardener manufactured by CibaGeigy.

c. BDMA is benzyldimethylamine accelerator available from E. V. Robertsor Ciba Geigy.

d. D32/4500 microspheres are borosilicate microspheres having a meandiameter of 75 micrometers, a specific gravity of 0.32, and acompressive strength of 4500 psi, available from the 3M Company.

e. GY70 fibers are graphite fibers milled to a length of about 150micrometers and having a diameter of about 8 micrometers, available fromthe Celanese Corporation.

f. KR38S, KR55, and KR9S are titanate coupling and wetting agents,available from Kenrich Petrochemical Company, Bayonne, N.J.

g. AF4 is a surfactant, available from Furane Chemical Co.

                  TABLE I                                                         ______________________________________                                        COMPOSITION OF FORMULATION S61                                                Component         PHR*    Weight (grams)                                      ______________________________________                                        1.    Resin                                                                         MY720 epoxy resin                                                                             100     400                                                   HY906 hardener  100     1.0                                                   BDMA accelerator                                                                              0.25    4.0                                                   KR38S           1.0     4.0                                             2     Microspheres                                                                  D32/4500        40      160                                                   KR55            0.3     1.2                                                   AF4 (Optional)  0.2     0.8                                             3     Fibers                                                                        Milled GY70     20      80                                                    KR9S            0.2     0.8                                             ______________________________________                                         *PHR is parts per hundred epoxy resin                                    

Preparation of Graphite Fibers

The GY70 graphite fibers in continuous tow form were cut into lengths ofapproximately 1/8 inch to 1/2 inch (0.32 to 1.27 centimeters), using apaper cutter. Batches of the chopped fibers (approximately 80 gramseach) were loaded into a ball mill jar having a one-gallon capacity andsufficient Freon TF was added to cover the ceramic balls to serve as asuspension medium. The fibers were milled for 24 hours. Scanningelectron micrographs of the milled fibers showed them to be broken intosmall fragments ranging from approximately 2 to 10 micrometers inlength.

The milled fibers and Freon were poured into a shallow stainless steelpan, and the Freon was allowed to evaporate. The fibers were then dried4 hours in an air-circulating oven set at 250° F. (121° C.) and siftedon a vibration plate to pass a 325 mesh screen. The dried, sifted fiberswere stored in a desiccator box until ready for use.

Composite Formation

The formulation S61 was prepared as follows. A one-gallon hot/cold potfor a Waring blender was heated to 140° F. (60° C.) using atemperature-controlled water bath. The premeasured amount of the HY906hardener was put in the blender and the mixer speed was adjusted using aVariac variable potentiometer so that the hardener was just barelyagitated. With the blender on "low" setting, the Variac was turned to 70percent of full speed. The resin, which had been preheated to 160° F.(71° C.), was added to the pot and the contents of the pot were mixeduntil the mixture appeared homogeneous (about 5 minutes), and thencooled to room temperature. Next, there was gradually added to the potthe KR38S, AF4 (optional), and 25 percent of the milled fibers which hadbeen previously dried overnight in an oven at 200° F. (93° C.) andfluffed by running in the blender on "low" speed at 70 percent of thefull Variac speed for about 15 seconds for 5 grams of fiber. The mixturewas mixed for about 5 minutes. Next, 10 percent of the microsphereswhich had been dried overnight in an oven at 200° F. (93° C.) wasgradually added and the contents of the pot were mixed until streaks ofmicrospheres disappeared. The remaining amount of fiber and the KR9Swere gradually added and the pot contents mixed for about 30 minutes.Next, the BDMA was added slowly, followed by the KR55 and the remainingamount of microspheres. The pot contents were mixed until streaks ofmicrospheres disappeared.

Then, the pot was covered and a vacuum pump was attached to the pot withthe pump set to pull a vacuum of 22 inches (559 mm) of mercury. Themixer was run for 45 minutes under vacuum or until there were no blackstreaks of fibers in the mixture. Finally, the mixture was carefullypoured so as to minimize air entrapment, into a preheated stainlesssteel test specimen mold which had been prepared by: cleaning withmethyl ethyl ketone solvent, baking at 300° F. (149° C.) for 30 minutes,brushing with a fluorocarbon mold release agent to provide three coatsof the release agent with 30 minutes air drying for each coat, andpreheating to 140° F. (60° C.). (Optionally, the formulation wasinjected with an air gun into the mold.) After pouring the mixture intothe mold, the mold was vibrated on a vibrating table for 5 minutes atthe maximum safe speed, with a large, flat, 0.5 inch thick aluminumplate placed on top of the mold. Next, the mold was placed in an ovenpreheated to 275° F. (135° C.) and a thermocouple was placed on/in eachof the following: on the mold, in the oven, and in the mold contentsthrough a hole in the side wall of the mold. When the thermocouple inthe mold contents registered 275° F. (135° C.), the following cure cyclewas run: 10 minutes at 275° F. (135° C.); 10 minutes at 300° F. (149°C.); 120 minutes at 350° F. (177° C.). The maximum oven rate was usedfor changing temperatures.

The mold was removed from the oven and was disassembled, and the partwas removed from the mold while the mold was still hot, being sure tokeep the thermocouple embedded in the syntactic foam. The part wasdeflashed as necessary with a file. For the post-cure, the demolded partwas placed in an oven preheated to 400° F. (204° C.) between 0.5 inchthick aluminum plates, with 2-5 kilograms weight on the top plate. Whenthe thermocouple in the syntactic foam registered 400° F. (204° C.), thefollowing post-cure cycle was run: 1 hour at 400° F. (204° C.); 1 hourat 425° F. (218° C.); 1 hour at 450° F. (232° C.), and 1 hour at 475° F.(246° C.) Finally, the part was removed from the oven.

The fiber reinforced syntactic foam composite formed as described abovewas found to have the properties shown in Table II. With regard to TableII, the following test requirements apply:

a. CTE was determined using a quartz dilatometer to measure the changein length as a function of temperature.

b. Specific gravity was measured using a pycnometer.

c. Viscosity was measured with a Brookfield Viscometer.

d. Shrinkage was measured by determining the dimensional differencebetween the molded product and the mold.

e. Gel time was determined qualitatively as the time required for theliquid resin to form a gel.

f. Pot life was determined qualitatively as the time required for theliquid resin to increase in viscosity to the point of being unworkable.

g. Degree of exotherm was determined by using a differential scanningcalorimeter.

                  TABLE II                                                        ______________________________________                                        PROPERTIES OF COMPOSITE OF S61 FORMULATION                                    Property           Value                                                      ______________________________________                                        CTE                19-22 × 10.sup.-6 cm/cm/°C.                                      10.6-12.7 × 10.sup.-6 in/in/°F.               Specific gravity   0.80                                                       Viscosity at 150° F. (65.6° C.)                                                    35,000 centipoise                                          Shrinkage          0.8%                                                       Gel time           >60 minutes                                                Pot life           >360 minutes                                               Degree of exotherm 15° F. (8.3° C.)                             ______________________________________                                    

EXAMPLE 2

This example illustrates a process for forming fiber-reinforcedsyntactic foam composites of various compositions which are related tothe present invention and which are described in copending patentapplication Serial No. (PD-81188), assigned to the present assignee.

The components of the various formulations designated as the "RSFseries" are shown in Table III. The following details regarding thespecific components apply to Table III.

a. Epoxy is a mixture of 70 parts Glyamine 135 (diglycidyl orthotoluidine) and 30 parts Glyamine 120 (tetraglycidyl methylenedianiline), both materials obtained from FIC Resins of San Francisco,Calif., mixed with about 115 parts nadic methyl anhydride hardener andabout 0.25 parts benzyldimethylaniline accelerator.

b. Zeeospheres 0.8 are solid glass spheres having a median diameter of 3micrometers, available from Zeelan Industries of St. Paul, Minn.

c. Carbospheres Type A are hollow carbon spheres having an averagediameter of 50 micrometers, available from Versar of Springfield, Va.

d. 3M A 32/2500 glass bubbles are glass microspheres having a meandiameter of 50 micrometers, a specific gravity of 0.32, and acompressive strength of 2500 psi, available from the 3M Company ofMinnesota.

e. 3M A 16/500 are glass microspheres having a mean diameter of 75micrometers, a specific gravity of 0.16, and a compressive strength of500 psi, available from the 3M Company.

f. Eccospheres SI are hollow silica microspheres having a diameter of45-125 micrometers, available from Emerson and Cuming Inc. of Canton,Mass.

g. Grefco 213 R40 beads are solid glass microspheres having a diameterof 3-8 micrometers, available from Grefco Inc. of Torrance, Calif.

h. HM-S 50 (50μ) are graphite fibers having a length of about 50micrometers and a diameter of about 8 micrometers, available fromCourtaulds Co. of the United Kingdom.

i. AS 50 (250μ) graphite are graphite fibers having a length of about250 micrometers and a diameter of about 8 micrometers, available fromCourtaulds of the United Kingdom.

j. 0.063" HMS-50 (1/16") are graphite fibers having a length of about1600 micrometers and a diameter of about 8 micrometers, available Finnand Fram of Sun Valley, Calif.

Using each of the formulations of the RSF series designated in TableIII, a fiber-reinforced syntactic foam composite was formed followingthe general procedure set forth in Example 1. The properties of each ofthese composites is shown in Table IV. The following test requirementswere applied for the measurements in Table IV.

a. Density was determined by pycnometer.

b. CTE was determined using a quartz dilatometer to measure the changein length (Δl) as a function of temperature.

c. Compressive strength was determined using the American Society forTesting and Materials (ASTM) Standard No. D695.

d. Compressive modulus was determined using ASTM D695, using crossheadspeed in place of strain guages.

e. Uniformity was determined by visual inspection.

f. Viscosity was measured with a Brookfield Viscometer.

                                      TABLE III                                   __________________________________________________________________________    COMPOSITION OF FORMULATIONS OF RSF SERIES                                                VOLUME RATIO OF FOAM FILLERS                                                  MICROSPHERES                                                       RSF-       ZEEO- CARBO-                    GREFCO                                                                             FIBERS                        FORMU-     SPHERES                                                                             SPHERES                                                                              3M A32/2500                                                                             ECCOSPHERES                                                                            213 R 40                                                                           HM-S 50μ                                                                          0.063" HMS-50          LATION                                                                              EPOXY                                                                              0/8   TYPE A GLASS BUBBLES                                                                           SI       BEADS                                                                              GRAPHITE                                                                             (1/16")                __________________________________________________________________________     3    0.405                                                                              0.098        0.471                   0.025                          4    0.375                                                                              0.048        0.514                   0.064                          5    0.401             0.500              0.059                                                                              0.040                          6    0.401             0.500              0.059                                                                               0.034*                        7    0.536                                                                              0.057        0.400                          0.007                   8    0.530                                                                              0.057        0.395                          0.018                  13    0.536                                                                              0.057        0.400                   0.007                         14    0.530                                                                              0.057        0.395                   0.018                         19    0.37 0.05         0.51                    0.06                          20    0.322                                                                              0.041                  0.586         0.050                         21    0.375                                                                              0.048        0.514                   0.064                         23    0.583                                                                              0.023 0.368                          0.026                         25    0.503                                                                              0.022 0.450                          0.025                         26    0.583                                                                              0.023 0.368                          0.026                         28    0.583                                                                              0.023 0.368                          0.026                         29    0.583                                                                              0.023 0.368                          0.026                         31    0.496                                                                              0.022        0.457                   0.025                         33    0.503                                                                              0.022                  0.450         0.025                         34    0.394                                                                              0.026        0.550                   0.030                         .sup. 34F                                                                           0.410                                                                              0.025        0.530                   0.035                         35    0.353                                                                              0.028         0.588**                0.032                         36    0.383                                                                              0.026                  0.560         0.031                         __________________________________________________________________________     *Plus 0.007 of AS 50 (250μ) graphite                                       **3M A16/500 used in place of A32/2500                                   

                                      TABLE IV                                    __________________________________________________________________________    PROPERTIES OF COMPOSITES OF FORMULATIONS OF RSF SERIES                                                COMPRESSIVE                                                                            COMPRESSIVE                                  RSF-      DENSITY                                                                             CTE     STRENGTH MODULUS  UNIFORMITY                          FORMULATION                                                                             (g/cc)                                                                              (10.sup.-6 in/in/°F.)                                                          (psi)*   (10.sup.3 psi)*                                                                        (1-10)  VISCOSITY                   __________________________________________________________________________     3        0.898 16.82   15,300   394      3.7     5                            4        0.881 13.81   16,300   447      4.3     4                            5        0.872 15.10   16,400   406      4.0     4                            6        0.869 22.16   14,300   407      3.5     5                            7        0.968 20.82   15,100   394      2.7     8                            8        0.982 21.39   18,400   410      4.7     7                           13        1.000 25.46   14,900   386      2.3     7                           14        1.019 25.55   15,400   405      1.8     6                           19        0.852 14.06   13,200   411      4.0     3                           20        0.694 14.09    8,600   335      2.7     2                           21         0.8561                                                                             17.02   14,000   439      3.4     3                           23         0.9912                                                                             20.69   16,300   384      5.5     7                           25         1.0387                                                                             30.51   19,000   423      --      6                           26        1.005 21.73   15,700   395      7.3     5                           28        0.982 20.10   17,100   411      6.5     5                           29        1.002 20.70   17,800   393      7.6     5                           31        0.888 23.90   17,800   400      6.6     8                           33        0.815 23.10   13,300   343      3.9     7                           34        0.824 14.59   17,500   394      4.4     6                           .sup. 34F 0.842 14.24   17,300   425      --      --                          35        0.738 17.23   10,200   303      4.0     6                           36        0.745 17.14   12,100   335      3.0     5                           __________________________________________________________________________     *1 psi = 1.45 × 10.sup.-4 pascals                                  

EXAMPLE 3

This example illustrates the formation of a fiber-reinforced syntacticfoam composite of the present invention using the preferred epoxy resinformulation and preferred vacuum liquid transfer molding processdescribed herein.

The heat curable epoxy resin formulation was prepared as disclosed incopending patent application Serial No. (PD-82339) and had the followingcomposition:

    ______________________________________                                        Resin Component    WT. (gms.)                                                 ______________________________________                                        Diglycidyl orthotoluidine                                                                        100                                                        Nadic methylanhydride                                                                            100                                                        2-ethyl-4-methyl imidazole                                                                        2                                                         ______________________________________                                    

This composition had a gel time of 25 minutes, a viscosity of 220centipoise at 75° F. (24° C.), and a CTE of 30.8 to 32.3×10⁻⁶ in/in/°F.(55.8 to 58.1×10⁻⁶ cm/cm/°C.).

A filler mixture was prepared having the composition shown below and adensity of 0.543 gm/cm³. Carbospheres are hollow carbon microballoonshaving a mean diameter of about 50 micrometers, available from VersarInc. of Springfield, Va. HM-S graphite fibers are graphite fibers havinga length of about 50 micrometers, available from Courtaulds Co. of theUnited Kingdom. Titanate sizing agents are available from KenrichPetrochemical Co. of Bayonne, N.J.

    ______________________________________                                        Filler Component    WT. (gms.)                                                ______________________________________                                        Carbosphere, 50 micrometers                                                                       50                                                        HM-S fiber, 50 micrometers                                                                        50                                                        Titanate sizing agent                                                                              1                                                        KR238M                                                                        ______________________________________                                    

Using the above-noted resin and filler, each of a series of resin/fillerformulations shown in Table V was processed as described below in orderto form the composite of the present invention.

The filler composition (i.e. a mixture of the fibers and microspherespretreated with the sizing agent as previously described herein) wasloaded into a cleaned 5.5 inch×0.5 inch (14 cm×1.3 cm) wide slab moldinternally coated with a polyvinyl alcohol release agent. The mold waspreheated to 212° F. (100° C.), the temperature at which hardening ofthe heat curable epoxy resin formulation was initiated. The epoxy resinformulation was poured into the mold containing the filler. The mold wasplaced in a laminating press, a nylon vacuum bag was constructed aroundthe compression tooling of the press, and a vacuum pressure of 125millimeters (mm) mercury pressure (166,625 pascals) was maintained onthe assembly for 2 minutes to draw down the resin to impregnate thefiller and to degas the resin materials in the mold. The vacuum was thenreleased without removal of the vacuum bag and the assembly held in thispassive vacuum state for an additional 2 minutes. Thereafter, a constantpositive pressure of approximately 800 pounds per square inch (41,360 mmHg or 5.5×10⁶ pascals) was imposed on the resin/filler mixture in themold for 2 hours at 212° F. (100° C.). During this pressurization stage,the resin was bled from the mold in the amount noted in Table V. Themolded composite slab had sufficient green strength to be ejected fromthe mold, whereafter it was post cured for 4 hours unrestrained, in anoven set at 300° F. (149° C.). The final void-free slab contained thefiller ratio noted in Table V and was cut into appropriate shapes forphysical testing. The composite was found to have the physicalproperties which are summarized in Table VI. As indicated by the valuesfor CTE given in Table VI, unexpected significant improvement in the CTEof the composites of the present invention was obtained using thepreferred resin composition and filler compositions described herein ascompared to the CTE values of the composites of Examples 1 and 2 herein.

In addition, a typical sample was tested in accordance with ASTME-595-77 and found to have a collected volatile condensible material(CVCM) of less than 0.1 percent and a total mass loss (TML) of less than1 percent, which meets the NASA outgassing requirements.

Further, for Specimen 1 of Table V, a portion of the molded slab wassurface plated with copper by subjecting the surface of the slab to anoxygen rich plasma treatment, as previously described. The treated slabwas then dipped into Shipley #328, electroless copper plating solution,as previously described, and then dried at 248° F. (120° C.) under 29inches (737 mm) Hg (guage pressure).

The plated composite was then evaluated for adhesion of the depositedcopper layer using an ASTM D3359 tape adhesion test before and after 25cycles of thermal shock imposed on the plated surface by alternatelydipping the plated specimen in liquid nitrogen (-320° F. or -196° C.)for 30 seconds and boiling water (212° F. or 100° C.) for 10 seconds. Noloss of copper was observed.

                  TABLE V                                                         ______________________________________                                        COMPONENTS OF MOLDING COMPOSITION                                                                      Resin                                                                         Bleed                                                                         During                                               Specimen                                                                             Resin    Filler  Filler Ratio Molding                                  No.    (gms.)   (gms.)  (Wt. %) (Vol. %)                                                                             %                                      ______________________________________                                        1      40       10      45      64     69.2                                   2      30       9.4     38      57     47.8                                   3      30       9.4     38      57     47.9                                   4      17.0*    5.5     37      57     45.3                                   5      19.0*    6.0     39      59     50.6                                   6      20.7*    6.5     41      60     54.6                                   ______________________________________                                         *KR134S sizing agent was substituted for the previously noted sizing          agent.                                                                        ##STR1##                                                                      where W.sub.1 = initial resin weight                                          W.sub.2 = resin displaced from the mold, using a bleeder cloth.               Vol. % calculated from resin bleed varies about 10-20% of the actual vol.     % value.                                                                 

                  TABLE VI                                                        ______________________________________                                        PHYSICAL PROPERTIES OF MOLDED COMPOSITES                                                                    CTE*                                            Specimen Thickness   Density  10.sup.-6 cm/cm/°C.                      No.      (in.)       gm/cm.sup.3                                                                            (10.sup.-6 in/in/°F.)                    ______________________________________                                        1        0.478       0.743     6.6 (3.6)                                      2        0.550       0.900    16.2 (9.0)                                      3        0.565       0.876    14.2 (8.1)                                      4        1.070       0.889    --                                              5        1.042       0.915    --                                              6        1.065       0.914    --                                              ______________________________________                                         *Determined using a quartz dilatometer.                                  

EXAMPLE 4

This example illustrates the formation of composites of the presentinvention as set forth in Example 3 with the exception that thecomposition of the filler formulation was varied. The procedure setforth in Example 3 was followed except that the filler compositionsshown in Table VII were used. The following details regarding thespecific components apply to Table VII.

a. Carbospheres are carbon microspheres having a specific gravity of0.32 and a mean diameter of 50 micrometers, available from VersarCorporation.

b. HM-S 50 (50μ) graphite fibers are graphite fibers having a length ofabout 50 micrometers and a diameter of about 8 micrometers, availablefrom the Courtaulds Co. of the United Kingdom.

c. 1/4 mm HM-S 50 graphite fibers are graphite fibers having a length ofabout 250 micrometers and a diameter of about 8 micrometers, availablefrom the Courtaulds Co. of the United Kingdom.

d. C15/250 glass microballoons are composed of borosilicate glass, havea diameter of 10-200 micrometers, a density of 0.15 gm/cm³, and acompressive strength of 250 psi, available from the 3M Company ofMinnesota.

The physical properties of the molded composite slabs so formed are setforth in Table VIII.

                                      TABLE VII                                   __________________________________________________________________________    COMPOSITION OF FILLER FORMULATIONS                                                                             C15/250                                                                       glass                                                                              Titanate                                         Carbosphere                                                                           HM-S 50 (50μ)                                                                      1/4 mm HM-S 50                                                                        micro-                                                                             Sizing Agent                            Filler                                                                            Density                                                                            Microballoons                                                                         graphite                                                                              graphite                                                                              balloons                                                                           KR238M                                                                             KR55                               No. (gm/cm.sup.3)                                                                      (gms)   fiber (gms)                                                                           fiber (gms)                                                                           (gms)                                                                              (gms)                                                                              (gms)                              __________________________________________________________________________    A   0.543                                                                              50      50      --      --   1    --                                 B   0.328                                                                              20      15      --      10   --   0.5                                C   0.387                                                                              --      4.4     45.6    25   1    --                                 __________________________________________________________________________

                  TABLE VIII                                                      ______________________________________                                        PHYSICAL PROPERTIES OF MOLDED                                                 COMPOSITE SLABS                                                               Filler                                                                        Used in                                                                              Thick-           Filler Ratio                                                                            Resin Cu                                    Molded ness    Density  (Wt.  (Vol. Bleed Coating                             Sample (in.)   (gm/cm.sup.3)                                                                          %)    %)    %     Removed                             ______________________________________                                        A      0.443   1.050    39    59    51.0   ≦5%                         B      0.540   0.753    26-47 56-77 61.9   ≦5%                         C      0.850   0.948    27    54    16.2  5-15%                               ______________________________________                                    

In addition, the surfaces of the molded composite slabs of Table VIIIwere then subjected to plasma treatment under the following conditions:O₂ /inert gas source of approximately 1000 ml/minute, vacuum pressure of200μ Hg, and one hour duration. The surfaces of the plasma etched slabswere then copper plated to a thickness of about 3-4 mils by dipping theetched slabs in an aqueous Shipley #328 electroless copper plating bath.

The plated composite was then evaluated for adhesion of the depositedcopper layer using the ASTM D3359 tape adhesion test and thermal shockcycle of Example 3. The adhesion results are recorded in Table VIII,indicating the amount of copper coating on lattice removed by the tape.

EXAMPLE 5

A syntactic foam composite prepared from Specimen No. 2 of Table Vdescribed in Example 3 and molded in a slab mold was plated with silveras follows. The surface of the slab was subjected to an oxygen richplasma etch which resulted in the removal of the surface polymer "skin,"as previously described. The etched slab was then metallized using theShipley Company #328 electroless copper plating solution process, aspreviously described, and thoroughly rinsed and dried at 248° F. (120°C.) under 29 inches (725 mm) Hg (gauge pressure), to provide a layer ofelectroless copper 20 microinches (5.08×10⁻⁵ cm) thick. Next, the slabwas immersed in an acid copper electrolytic plating bath at 25° C. for25 minutes to form an electrodeposited copper layer 100 microinches(2.54×10⁻⁴ cm) thick. Finally, the copper-plated slab was immersed in anelectrolytic silver plating bath at 25° C. for 25 minutes to form alayer of silver 300 microinches (7.62×10⁻⁴ cm) thick.

The silver-plated slab was then evaluated for adhesion of the depositedlayer using an ASTM D3359 tape adhesion test before and after 25 cyclesof thermal shock imposed on the plated surface by alternately dippingthe plated specimen in liquid nitrogen (-328° F. or -196° C.) for oneminute and boiling water (212° F. or 100° C.) for one minute. Noadhesion loss of silver was observed.

The silver-plated syntactic foam had the same low R.F. losscharacteristics as aluminum when tested for insertion loss at 4.6gigahertz using standard electronic tests. Thus, with the use of propertooling for molding, antenna waveguide structures may be formed from thecomposite of the present invention, which are effective microwave orantenna components and which meet the requirements for use in spaceapplications. Syntactic foams plated with metals such as silver andcopper may serve as metal-plated core materials for both microwavecomponents and microwave reflectors.

The fiber-reinforced syntactic foam composites of the present inventionachieve a 3-to-1 reduction in weight in comparison with aluminum, whichmakes these components attractive for weight-sensitive applications in aspacecraft environment. At the same time, however, in situations callingfor high volume production, the readily-moldable nature of thereinforced foam mixture disclosed herein further offers the potential ofsignificantly reduced cost in comparison with the machiningtraditionally employed for the production of conventional metal parts.

The preceding description has presented in detail exemplary preferredways in which the concepts of the present invention may be applied.Those skilled in the art will recognize that numerous alternativesencompassing many variations may readily be employed without departingfrom the intention and scope of the invention set forth in the appendedclaims. In particular, the present invention is not limited to thespecific resin, fibers, or microballoons set forth herein as examples.By following the teachings provided herein relating to the effect ofeach component of the mixture on the final composite and the effect ofthe various components on each other, other suitable resin, fiber, andmicroballoon materials may readily be determined. Further, by followingthe teachings provided herein, it may be determined how to formcomposite materials having a density or coefficient of thermal expansionother than those set forth herein as required for the specificallymentioned end use in space applications.

What is claimed is:
 1. A fiber-reinforced syntactic foam compositehaving a specific gravity less than 1.0 and a coefficient of thermalexpansion of about 9.0×10⁻⁶ in/in/°F. (16.2×10⁻⁶ cm/cm/°C.) or less, thecomposite being prepared from an admixture of:(a) a heat curablethermosetting resin comprising: an uncured polyglycidyl aromatic amine,a polycarboxylic acid anhydride curing agent, and a curing acceleratorselected from the group consisting of substituted imidazole compoundsand organometallic compounds; (b) hollow microspheres having a diameterin the range of about 5 to about 200 micrometers; and (c) fibers havinga length of less than or equal to 250 micrometers.
 2. The composite ofclaim 1 wherein the polyglycidyl aromatic amine is diglycidylaniline. 3.The composite of claim 1 wherein the polyglycidyl aromatic amine isdiglycidyl orthotoluidine.
 4. The composite of claim 1 wherein thepolyglycidyl aromatic amine is tetraglycidyl metaxylylene diamine. 5.The composite of claim 1 wherein the polycarboxylic acid anhydride ispresent in sufficient quantity to react with from about 60 to about 90percent of the epoxide groups in said polyglycidyl aromatic amine. 6.The composite of claim 1 wherein the polycarboxylic acid anhydride isnadic methyl anhydride.
 7. The composite of claim 1 wherein thepolycarboxylic acid anhydride is methyl tetrahydrophthalic anhydride. 8.The composite of claim 1 wherein the polycarboxylic acid anhydride ismethyl hexahydrophthalic anhydride.
 9. The composite of claim 1 whereinsaid curing accelerator is present in the amount of about 0 to about 3percent, by weight.
 10. The composite of claim 1 wherein said curingaccelerator is 2-ethyl-4-methyl imidazole.
 11. The composite of claim 1wherein said curing accelerator is stannous octoate.
 12. The compositeof claim 1 wherein:(a) said uncured polyglycidyl aromatic amine isdiglycidyl orthotoluidine and is present in the amount of about 100parts per hundred resin by weight; (b) said curing agent is nadic methylanhydride and is present in the amount of about 100 parts per hundredresin by weight; and (c) said curing accelerator is 2-ethyl-4-methylimidazole and is present in the amount of about 2 parts per hundredresin by weight.
 13. The composite of claim 1 wherein the hollowmicrospheres are formed of a material selected from the group consistingof glass, silica, carbon, acrylate resins, and phenolic resins.
 14. Thecomposite of claim 12 wherein the hollow microspheres are formed ofglass and have an average diameter of about 50 micrometers.
 15. Thecomposite of claim 12 wherein the hollow microspheres comprise a mixtureof glass microspheres and carbon microspheres.
 16. The composite ofclaim 1 wherein the fibers are formed of a material selected from thegroup consisting of graphite, glass, carbon, nylon, and polyamide. 17.The composite of claim 16 wherein the fibers are formed of graphite andhave a length of about 50 micrometers and a diameter of about 8micrometers.
 18. The composite of claim 1 wherein said admixture furthercomprises solid microbeads.
 19. The composite of claim 1 wherein saidadmixture further includes a coupling and wetting agent.
 20. Thecomposite of claim 1 which comprises about 35 to about 65 percent byvolume microspheres and about 3 to about 10 percent by volume fibers,the balance being a matrix comprised of the heat cured resin throughoutwhich the microspheres and fibers are dispersed and bonded together. 21.The composite of claim 20 which additionally comprises about 2 to about8 percent by volume of solid microbeads having a diameter of about 2 toabout 8 micrometers.
 22. A fiber-reinforced syntactic foam compositehaving a specific gravity less than 1.0 and a coefficient of thermalexpansion of about 9.0×10⁻⁶ in/in/°F. (16.2×10⁻⁶ cm/cm/°C.) or less,said composite comprising:(a) a heat curable thermosetting epoxy resincomprising:(1) diglycidyl orthotoluidine in the amount of about 100parts per hundred resin by weight; (2) nadic methyl anhydride in theamount of about 100 parts per hundred resin by weight; and (3)2-ethyl-4-methyl imidazole in the amount of about 2 parts per hundredresin by weight; (b) hollow carbon microspheres having a diameter in therange of about 20 to about 200 micrometers; and (c) graphite fibershaving a length of less than or equal to 250 micrometers and a diameterof about 8 micrometers.
 23. An article of manufacture comprising a bodyformed from the composite material of claim
 1. 24. The article ofmanufacture set forth in claim 23 which further comprises a layer of anelectrically conductive material adhered to selected surfaces of thebody.